The present invention is directed generally to reinforced concrete structures, and more particularly to pretensioned concrete structures such as beams and other load-bearing members wherein the structures are placed in initial compression in order to increase their flexural load bearing capacity.
Pretensioned concrete structures have been formed by casting concrete around pretensioned steel cables. When the concrete has set, the tension on the steel cables is released and the consequent relaxation of the cables imparts a compressive load on the concrete structure. This initial compressive set allows the structure to react more flexural load because the tensile stress developed at maximum loading is reduced in accordance with the amount of initial compression.
A conventional prior art method and apparatus for fabricating pretensioned concrete beams is illustrated in FIGS. 1-3. A pretensioning apparatus 10 is formed from a tensioning bed 12 and a pair of anchor blocks 14 and 16 at each end of the tensioning bed 12. Each anchor block has one or more cables bores therein. Positioned at one end of the tensioning bed 12 is a cable anchor 18. Positioned at the opposite end of the tensioning bed 10 is a cable tensioning jack 20. In order to fabricate a pretensioned concrete beam, one or more cables 22 are mounted between the anchor 18 and the jack 20 and pretensioned using the jack.
Still referring to FIGS. 1-3, a pair of steel concrete form structures 24 and 26 (see FIG. 3) are positioned on the tensioning bed 12 with the cables 22 therebetween, and one or more concrete beams 28 are poured. When the concrete has set, the cables 22 are released from the anchor 18 and jack 20 and the cables are cut between the beams 28. Each beam is thus pretensioned by the cable members trapped therein.
It will be appreciated that the above-described prior art method and apparatus produce pretensioned concrete beam members in which the pretensioning is uniform along the length of the beams and in which the cables extend along a linear path through the beams. Those characteristics effectively limit the length of the beams that are producible in accordance with the prior art, for two reasons. First, uniform tension along the beam length means that for flexural structures the tension can be too high at the ends of the beam, where bending is often minimal, and too low at the center of the beam, where maximum bending usually occurs. Second, a linear cable path means that the cables do not follow the load curve of the beam. Thus, the amount of pretensioning may be excessive at some locations on the beam and insufficient at others. This limits the maximum length of prior art pretensioned concreted structures to approximately 100 feet.
Accordingly, there is an evident need for a system for fabricating pretensioned concrete structures that overcomes the deficiencies of the prior art. A new technique is required that would allow the formation of concrete beams and other structures well in excess of the beam lengths allowed by the prior art, and in which efficiencies are maximized by matching the pretensioning more precisely to the loading pattern of the structure.